A field-effect transistor (FET) and a single-electron transistor (SET) are elements that convert voltage signals input in a gate into current signals output from either a source electrode or a drain electrode. On placing a voltage between the source electrode and the drain electrode, charged particles existing in the channel move between the source electrode and the drain electrode along the direction of electric field and are output from either the source electrode or the drain electrode as a current signal.
At this point, the strength of the output current signal is proportional to the density of the charged particles. When a voltage is applied on the gate that is placed at upward, sideward or downward position of the channel with an insulator therebetween, the density of the charged particles existing in the channel is changed. With the aid of this property, the current signal can be varied by changing the gate voltage. Hereinafter, a field-effect transistor and a single-electron transistor are both called simply a “transistor” when they are not necessary to be distinguished from each.
The currently known chemicals-sensing elements (sensors) using transistors are those utilizing the above-mentioned principles of transistors. As a specific example of sensors, the one described in Patent Document 1 can be mentioned. Patent Document 1 discloses a sensor with construction that a substance which is capable of selectively reacting with detection targets is immobilized on the gate of the transistor. A change in the surface charge of the gate, induced by the reaction of the detection targets and the substance immobilized on the gate, varies the electric potential of the gate, thereby the density of the charged particles existing in the channel being changed. This change leads to the variation in the output signal from either the drain electrode or the source electrode of the transistor. Then the detection of a detection target can be made by reading that variation.
[Patent Document 1] Japanese Patent Laid-Open Publication (Kokai) No. Hei 10-260156